In the mechanical realization of some electronic circuits, such as those employed in large scale digital computers, significant quantities of circuitry may be packaged together in a module known in the art as a micropackage. Within a typical micropackage, one surface of a good thermally conductive (such as alumina) substrate supports multiple layers of circuit paths, and a top layer receives one or more integrated circuits interconnected to perform the electronic office of the module. The module is energized by contacts bearing against certain areas of the module cover structure, and interconnection to other modules is accomplished through numerous contact areas peripherally distributed about the edges of the module. Those skilled in the art will understand that the module is sealed against contamination from the environment of use and that cooling is effected by heat transfer from the integrated circuits across the circuit layers to the substrate whose outer face forms one major surface of the module. During module operation, this outer face is placed into heat transfer relationship to a heat sink system. In normal operation within an operating digital computer or the like, the heat sink system typically includes a flexible copper diaphragm which lays against the outer substrate face, the other side of the diaphragm being bathed by circulating cooling fluid such as water. The cooling fluid is maintained at above atmospheric pressure such that the diaphragm conforms to any slight non-planar regions of the surface area of the outer face of the substrate, thus ensuring good thermal transfer from the substrate to the circulating fluid. In a well-designed electronic system employing such micropackages and such a cooling technique, densely populated, heat generating micropackages may operate trouble free for many years because of the effectiveness of the cooling methodology.
At the end of their fabrication, each micropackage must be thoroughly tested individually to ensure that it fully meets its operating specifications. In addition, it is desirable to similarly test micropackages which have failed in the field to determine the reason for its failure. These tests are carried out by mounting a given module in a test fixture and using a special purpose tester which can exercise the module through all its states and log the results. Typically, the test fixture incorporates a massive heatsink to simulate the normal operation cooling system described above, the outer face of the substrate bearing against a planar surface of the heatsink.
In the performance of these tests, an unexpected and potentially destructive heat transfer condition has been encountered. Generally, the problem has been observed as a substantial difference between the temperature of the substrate in the center area as opposed to the temperature around its periphery, the former being much higher. Those skilled in the art will recognize that this condition indicates a decidedly non-uniform heat transfer condition between the substrate and the planar surface of the heatsink. In order both to protect a micropackage under test from damage or destruction from localized overheating and to ensure valid test results, it is highly desirable that the heat transfer between the outer face of the micropackage substrate and the planar surface of the heat sink be as uniform across the interface as possible, and it is to that end that this invention is directed.